Pics?
How old were they before you changed the light schedule from veg to flowering?
Anyway, if some of them were showing preflowering then the others probably weren't far behind. I wouldn't worry. You`re right about your concern for headroom IMO.
You shouldn't have to stake indicas at this stage- sounds like they`ve stretched for some reason. How close was the MH to the plants? 400w should be plenty. what size pots are they in?
4`x4` isn't loads of space for 15 plants.
If they're wilting when you water them then they're probably getting overwatered. Keep the soil moist but not soaking.
I`d say you should be able to determine sex within 2 weeks of putting them under 12 on 12 off lighting.
P.S. Please use paragraphs
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